1. Field of the Invention
This invention relates to a method for the removal of nitrogen oxides from an exhaust gas of a diesel engine. More particularly, it relates to a method for the removal by catalytic reduction in the presence of ammonia of nitrogen oxides (hereinafter referred to as "NO.sub.x ") which are contained in the exhaust gas from such diesel engine automobiles as trucks and buses, the exhaust gas from diesel engine generators, and the exhaust gas from stationary diesel engines, for example. Especially, this invention relates to a method for the removal of NO.sub.x by the contact of the exhaust gas with a honeycomb catalyst in the presence of ammonia, which method is such as to enable the removal of NO.sub.x to be effected efficiently in real-time conformity with the variation in the behavior of the exhaust gas and, at the same time, repress the inclusion of ammonia in the exhaust gas departing from the site of NO.sub.x removal.
2. Description of the Prior Art
Heretofore, as a purifying catalyst for the exhaust gas emanating from an internal combustion engine, particularly a gasoline engine automobile, the so-called three-way catalyst capable of simulataneously removing NO.sub.x, CO (carbon monoxide), and HC (hydrocarbons) from the exhaust gas has been widely used.
The three-way catalyst functions most effectively while the gasoline engine is being operated under a reducing atmosphere with the air-to-fuel (A/F) ratio kept in the proximity of the chemical equivalent point. While the automobile car is in motion, therefore, the gasoline engine is generally controlled with an electronically controlled injection device, for example, to maintain the A/F ratio at a constant level. Studies have been continued to date with a view to developing a catalyst which effects efficient removal of NO.sub.x, CO, and HC under the conditions permitting maintenance of this constant A/F ratio. So far, numerous catalysts using such precious metals as platinum, palladium, and rhodium have been proposed.
In the case of a diesel engine which is likewise an internal-combustion engine, however, the exhaust gas has a very high oxygen concentration in the range of 4 to 20% and remains in an oxidizing atmosphere of an oxygen-rich gas composition. When the three-way catalyst is used for the removal of NO.sub.x from this exhaust gas, it has been known that the removal of NO.sub.x is attained only with great difficulty because the CO and HC components in the exhaust gas intended to serve as reducing agents are spent out by oxidation.
Since diesel automobiles enjoy a high fuel efficiency, they tend to grow quantitatively in the future. Since the removal of NO.sub.x from the exhaust gas of diesel automobiles is very difficult for the reason given above, the measure concerning the NO.sub.x from diesel motor cars poses itself a serious social issue.
Heretofore, among various means for the removal of NO.sub.x under an oxidizing atmosphere, the method of selectively reductive removal of nitrogen oxides using ammonia (NH.sub.3) as a reducing agent has been accepted as one even under an oxidizing atmosphere in inducing the selective reaction of NO.sub.x with ammonia without being affected by the oxygen concentration in the exhaust gas. Thus, it has been widely applied to the purification of the exhaust gas from such stationary sources as boilers and furnaces in a thermal power plant, for example.
As a measure for controlling the flow volume of ammonia in the method of ammonia-selectively reductive removal of nitrogen oxides, there has been employed a method which comprises determining the total NO.sub.x content of the exhaust gas by multiplying the signal of the exhaust gas flow volume issuing from an exhaust gas flow volume detector by the NO.sub.x concentration signal issuing from an NO.sub.x concentration measuring device adapted to measure the NO.sub.x concentration in the exhaust gas, then determining the ammonia gas flow volume by multiplying the total NO.sub.x content mentioned above by the prescribed NH.sub.3 /NO.sub.x ratio, and feeding the resultant output signal to an ammonia flow meter to control the flow of ammonia gas in conformity with the variation in the NO.sub.x content of the exhaust gas due to the variation in the load such as of a boiler or a method which comprises detecting the concentration of leaking ammonia at the outlet of a reactor, synchronously actuating an ammonia flow meter so as to lower the concentration of leaking ammonia below a fixed level thereby controlling the amount of ammonia gas to be introduced to the site of reaction.
When the ammonia selective reduction method (SCR method) is employed for the treatment of the exhaust gas from such a mobile source as a diesel automobile, however, since the automobile is rarely driven at a fixed but is mostly operated by randomly repeating such actions as idling, acceleration, slowing, and deacceleration. The temperature, amount, and composition of the exhaust gas proportionately change by the minute. Particularly the NO.sub.x content in the exhaust gas is known to vary remarkably.
Where the NO.sub.x content of exhaust gas is remarkably varied as in the exhaust gas from a diesel automobile for example, it is by all means necessary that ammonia should be supplied in strict conformity with the NO.sub.x content. By the conventional method of ammonia control employed for the removal of the NO.sub.x of the exhaust gas from a stationary source, however, accurate control of the flow volume of ammonia is not easily obtained because the device for the determination of NO.sub.x concentration possesses a relatively slow response speed and there inevitably arises a considerable time lag between the time the NO.sub.x concentration at the inlet of the de-NO.sub.x unit and the ammonia concentration at the outlet of the de-NO.sub.x unit are detected and the time the feed amount of ammonia is adjusted and the feed of ammonia in the adjusted amount is started. As the result, it is extremely difficult to obtain effective removal of the NO.sub.x from the exhaust gas and, at the same time, repress to the fullest possible extent the amount of ammonia suffered to leak in the exhaust gas departing from the site of NO.sub.x removal.
As a means for lowering the NO.sub.x content of the exhaust gas from an internal-combustion engine, a method which effects the diminution of the NO.sub.x content by adding ammonia to the exhaust gas proportionately to the amount of the fuel consumed and passing the resultant mixed gas through a reactor packed with a pelletized catalyst, therein to be subjected to reducing combustion has been known to the art (U.S. Pat. No. 4,403,473). In the case of the method of this nature, it may be safely concluded that the NO.sub.x content in the exhaust gas and the amount of fuel consumed are substantially proportional to each other where the revolution number of the engine is constant- as illustrated in FIG. 1 of the specification of the U.S. patent. Where the revolution number and the torque of the engine simultaneously vary as in the automobile in motion, since the NO.sub.x content and the amount of fuel consumed are not in direct proportion to each other, it is difficult for this method to supply ammonia in strict conformity with the momentarily varying NO.sub.x content. Thus, this method cannot adapt itself to the removal of the NO.sub.x in the exhaust gas from a mobile source.
Any method suitable for efficient removal of the NO.sub.x from the exhaust gas of a diesel automobile in the presence of ammonia has not yet been developed for the reason given above.
An object of this invention, therefore, is to provide a method for the removal of nitrogen oxides from the exhaust gas of a diesel engine.
Another object of this invention is to provide a method for efficient removal by catalytic reduction of the NO.sub.x from the exhaust gas of a diesel engine in the presence of ammonia.
A further object of this invention is to provide a method for the removal of the NO.sub.x from the exhaust gas of a diesel engine automobile in motion by the contact of the exhaust gas with a catalyst in the presence of ammonia, which method permits the removal of the NO.sub.x to be effected efficiently for a long time in spite of any possible sharp variation in the behavior of the exhaust gas and, at the same time, represses to the fullest possible extent the leakage of ammonia in the exhaust gas departing from the site of NO.sub.x removal.